Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and suitable clear plastic bottles. SODIS is an inexpensive and effective method for decentralized water treatment, usually applied in domestic use and is recommended by the World Health Organization as a viable method for home water treatment and safe storage. SODIS is already applied in many developing countries.
Exposure to sunlight has been shown to inactivate diarrhea-causing organisms in contaminated drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms:
- UV-A directly interferes with bacterial metabolism and destroys the structures of microorganisms
- UV-A (wavelength 320-400 nm) reacts with dissolved oxygen in water and produces highly reactive forms of oxygen (oxygen that releases radicals and oxygenated waters), which also destroy pathogens.
- Infrared radiation heats up the water, preventing the growth of microorganisms.
Guidelines for application in domestic uses
- Transparent bottles are filled with water from contaminated sources. For oxygen saturation, the bottles can be three-quarters full, then shaken for 20 seconds, and then completely filled. Very cloudy water (turbidity higher than 30 NTU) must be filtered before exposure to sunlight.
- The filled bottles are then exposed to the sun. The best temperature effects can be achieved if the bottles are placed on a corrugated roof as compared to thatched roofs.
- The treated water can be consumed. The risk of new contamination can be minimized if the water is stored in bottles. The water should be consumed directly from the bottle or poured into clean drinking cups. Refilling and storage in other containers increase the risk of contamination.
SODIS is an effective method of treating water where fuel or stoves are unavailable or prohibitively expensive. Even where fuel is available, SODIS is a cheaper and less environmentally aggressive option. The application of SODIS is limited if there are not enough bottles available, or if the water is very cloudy.
In theory, the method could be used in disaster relief or refugee camps. However, supplying bottles can be more difficult than supplying the equivalent disinfection tablets containing chlorine, bromine, or iodine. Also, in some circumstances, it can be difficult to guarantee that the water will be left in the sun long enough.
There are other methods for home water treatment and safe storage, eg chlorine disinfection, different filtration procedures or flocculation / disinfection. Selection of the appropriate method should be based on efficacy criteria, treatment along with other types of contamination (turbidity, chemical contaminants), treatment costs, work input and convenience, and user preference.
If water bottles are not left in the sun for the proper amount of time, the water may not be safe to drink and could cause illness. If the sunlight is less strong, due to cloudy weather or less sunny weather, a longer exposure time in the sun will be necessary.
The following issues should also be considered:
- Bottle material: Some glass or polyvinyl chloride materials can prevent UV light from reaching the water. Commercially available bottles made of Polyethylene Terephthalate are recommended. Handling is much more convenient in case of suitable bottles. Polycarbonate blocks all UVA and UVB rays, and therefore should not be used.
- Aging of plastic bottles: the effectiveness of SODIS depends on the physical state of the plastic bottles, with scratches and other signs on the walls that reduce the effectiveness of SODIS. Very scratched or old bottles should be replaced with more modern ones.
- Container shape: the intensity of UV radiation decreases rapidly with increasing water depth. At a water depth of 10 cm and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. Regular soda bottles are often readily available and are more practical for the SODIS application.
- Oxygen: sunlight produces highly reactive forms of oxygen (oxygen releases radicals and oxygenated waters) in water. These reactive molecules contribute to the destruction process of microorganisms. Under normal conditions (rivers, creeks, wells, ponds, tap) the water contains sufficient oxygen (Oxygen of more than 3 mg per liter) and does not have to be aired before applying SODIS.
- Bottle Material Leaching - There was some concern about whether plastic from containers can release chemicals or toxic components into the water, a process possibly accelerated by heat. The Swiss Federal Laboratories for Materials Testing and Research have examined the diffusion of adipates and phthalates (DEHA and DEHP) from new and reused normal bottles into water during sun exposure. The concentration levels found in the water after a 17 hour sun exposure at 60 ° C the water were well below the values indicated by WHO for drinking water and in the same magnitude as the concentrations of phthalate and adipate generally found in high-quality tap water. Concerns about the general use of ordinary bottles were also expressed after a report published by researchers at the University of Heidelberg on antimony released from normal bottles for soft drinks and mineral water stored more than several months in supermarkets.
However, the antimony concentrations found in the bottles are orders of magnitude under WHO and national guidelines for antimony concentrations in drinking water. Furthermore, SODIS water is not stored for such long periods in bottles.
Health impact, diarrhea reduction
It has been shown that the SODIS method (and other home water treatment methods) can effectively remove pathogenic contamination from water. However, infectious diseases are also transmitted by other routes, that is, due to a general lack of sanitation and hygiene. Studies on the reduction of diarrhea among SODIS users show reduction values of 30-80%.
SODIS has also been applied in several communities in Brazil, one of them being Prainha do Canto Verde north of Fortaleza. There, the villagers have been purifying their water using the SODIS method. It is completely successful, especially since the temperature during the day can go beyond 40 ° C (100 ° F) and there is a limited amount of shade.
The new SODIS manual presents an overview of the SODIS method based on 20 years of research and practice.
It is structured in three parts:
SODIS at a glance: The first part presents the basic facts about solar disinfection and the SODIS method, provides information on the history of the SODIS method and highlights its benefits. The SODIS niche in the context of HWTS and WASH and the potential impact and role of SODIS in relation to programs aimed at reducing the prevalence of diarrheal diseases are also analyzed.
Technical aspects of the SODIS method: the second part deals with the technical aspects that influence the effectiveness of the SODIS method to eliminate pathogens from water. This information is intended to provide a solid understanding of the correct application of SODIS in the field, and the expected results under various conditions.
Promotion of the SODIS method:The third part reviews specific approaches and tools used to promote the SODIS method for target populations. Its objective is to support experts with the integration of the SODIS method in HWTS, WASH and health projects.
With information from: